Hello Ctj
Not quite sure here , I have the sister van to you The Sterling Elite , so here goes for the procedure on ours.
Right hand front under the bed is a Sargent Elecrical box, must be switched on there is a small window to the right of this box with a small button to go through settings press until tank fill appears and yes there should be an external pump to fill onboard tank and it will take about one and a half aqua rolls full to fill on board tank..
When the onboard tank is full your water supply will come from the onboard tank and it will call for water from the aqua roll as and when it needs it, this is ideal on a fully serviced pitch but more about that another time
If you want to know more and you are not sure call for a moderator to get my Email address and there I will go through it step by step or even give you my mobile and we will sort you out
Regards Sir Roger